Aircraft integral fuel tanks are so named because they are an integral part of the aircraft frame and skin. While use of irregularly shaped cavities of wings and fuselage as fuel tanks is a necessary and desirable utilization of space particularly in military aircraft, the problem of sealing the cavities so as to inhibit hazardous fuel leakage has been the subject of extensive research. Thus, the aircraft frame and skin must be sealed at every joint and fastener by a flexible material to provide a permanent barrier to fuel leakage. Fuel resistant elastomers, such as polysulfides, fluorosilicones, polyesters, cyanosilicones, and urethanes have been used as sealants with varying degrees of success.
Integral fuel tank sealants can be divided into three types, namely, (1) filleting, (2) faying, and (3) channel. Filleting sealants are viscous liquid polymer mixes which are extruded over seams, into joint corners, over fasteners, and around openings for hydraulic lines and electrical conduits. Generally, this type cures to a tough rubber-like material in a few days at room temperature. Repair is accomplished by physically removing the fillet and reapplying a fillet of the same or a compatible material. In inaccessible areas it is difficult to make repairs. Faying sealants are liquid elastomeric compounds, often thinned with a solvent, which are applied between skin and structural members. After application to the mating surfaces, the parts are fastened together. Most faying surface sealants are room temperature vulcanizing porducts that reach full state of vulcanization in a few days. It is necessary to disassemble the skin and frame in order to repair faying surface sealants. Channel sealants are non-vulcanizing mastics that are injected into grooves formed in the structure-skin joints. The sealants can be applied as the aircraft is assembled or injected through ports after assembly.
While any one of the three types of sealants described in the preceding paragraph can be used alone, to ensure reliability many aircraft often employ two or all three. When the non-curing channel sealant is utilized as the primary sealant for integral fuel tanks, a careful balancing of properties is required. The sealant must have an elastomeric flexibility, it must adhere to metal with a tacky-type adhesion, it must resist being pushed or extracted from the joint by fuel, and it must retain these properties over a wide temperature range, e.g., from -65.degree. F. to 350.degree. F. The most common failure mode for channel sealants is gap extrusion caused by pressure build-up in the channel from thermal expansion, fuel swell, internal tank pressures, and, to a lesser extent, joint flexing and gas formation from polymer degradation. In U.S. Pat. No. 3,580,870 a channel sealant composition is disclosed that includes a sealant medium containing plastic balls or spheres graduated in size from large balls to medium size balls to small balls. As described by the patentee, the balls congregate or cluster along gaps between the skin sheet and wall of a fuel tank, thereby preventing escape of sealant material. Under test conditions comparable to those encountered in aircraft operation, it has been found that the sealant composition containing plastic balls is ineffective in preventing fuel leakage.
It is a principal object of this invention, therefore, to provide a channel sealant composition which is effective in preventing fuel leakage from integral fuel tanks.
Another object of the invention is to provide a channel sealant having a composition such that loss of sealant through structural gaps through extrusion is minimized.
A further object of the invention is to provide a sealant composition which, when injected into a channel of an integral fuel tank, exerts a squeegee action in pushing out and completely replacing old sealant.